RTO Fan/Motor Base Vibration question

Hi,
Had some interesting data gained last night and could use some feedback on what it might indicate. This is on a pull through system. The fan is located by the exhaust stack of the oxidizer. It pulls air through a combustion chamber/ceramic media on a 3 tower unit.
In july 2004 the 400hp motor/fan were replaced with a 700hp motor/fan. The only duct modification was done to the breech section. The outlet duct was neither enlarger nor lengthened to accomodate the new equipment.

About 10 months ago a vibration sensor on a motor base started indicating the fan/motor was experiencing high vibration levels. This was happening when the VFD controlling the motor was running within a certain speed/freq range (80-85%). When operating at speeds above and below the range the vibration levels were good.

Initial inspections indicated that the vibration seemed to be coming from the breech side of the fan. Some insulation on the breech had fallen off (about 1/2 on one side of the square duct). However the breech didn't seem to vibrate.
In December of last year I trended the vibration levels at .5% incrememnts from 80%-86%. When the VFD was at 81% (49.2hz appromately) the unit experienced very high vibration levels

This past weekend an air straightener was added to the inlet to the fan.
When initially starting up the unit it ran at 25% until shortly after the burners in the chamber lit.
At this point I ramped it from 25% to 80% (manually at the vfd). The highest vibration level reached was 0.113ips. From this speed until I reached 85% the vibration level never exceeded .090isp.
It looked like the straightener had resolved the problem.

However at 85.5% (until I stopped at 88%) the vibration level got slightly higher (into the .130s range). While not much of a concern it was a different pattern than I had seen previously.
By this point the outlet duct and fan bearings were getting near normal operating temperatures (bearings 140s-170F, outlet between 150-220F).
To make sure I had accurate frequencies to tie into VFD speeds (I had noticed that the manual speed demand/freq were slightly off) I started recording the actual freq's for the various speeds).
When I went back through the 80-85% speeds I found that the vibration levels had increased dramatically. They weren't it the .085 to 0.09ips range any longer. They had dramatically increased and were hovering in the .300s-.525ips range.


It seems that temperature is a factor. I'm not sure if bearing temps or the outlet temp is the culprit. The fact that it's only occurring during certain speeds is also interesting.

While I can't take the heat out of the bearings, for testing purposes I can take it out of the outlet manifold/fan (by various means).


Has anyone seen anything like this before? I'd like any feedback.


The fan is by Twin City. It turns out the fan had been previously sold and returned. It was sold to it's current owner in 2004, without the curren owner's knowledge of its history. I've asked Twin City for the history of this fan, and while they promised to get it to me it has been over 6 months.
I've checked the fan for cone clearance, loose bolts, cracks in the wheel. Haven't found anything conclusive. The only I have found is that I'm tightening motor base anchor bolts every few months. Which is unusual. But with the vibration issue it doesn't seem out of place.

Something is out of balance, most likely the fan blade.

I waited for someone to respond. Its either the blade, bearings, or mounting...ASSUMING you repaired any other relevant conditions that are external to the motors settings.

You do have installation vibration measurements over the range it was expected to run? If so and the deviation is significant then something has changed, can be external or internal in relevance to the motor.

Thanks for the replies. I agree that something has changed. The two questions left are what is it and why does it seem temperature dependent?
When I first trended the fan yesterday the vibration levels were beautiful. However 20 minutes after trending began (after I had already gone through all the 'critical' speeds with great levels) the whole thing went to pot. The only noticeable difference were: 1) temp at the fan (the air temp had reached close to operating range), and 2) fan bearing temps.

Could expansion at the fan cause this phenomenon at certain speeds?

Expansion, maybe but my money would be bearings. Grease or whatever can gel when cool but kind of liquify when hot, that and ANY weakness/wear of the bearing(s) could (would actually) show itself in vibrations.

I assume you can lubricate the bearings, did you at some point in the tests?

Very interesting point. The bearings have a stated requirement of .8 gallons per minute flow per bearing. Unfortunately my former employer left the original lube unit in place (a sturdy RHM unit btw for any that care) but it is only rated for .5 gallons per minute total flow. Therefore only .25 gallons per minute to each bearing.
From the initial start the bearings ran 'hotter' than what I was used to. I was expecting a 140-150s range, and got 170s-180s... which manifested itself as high lube temp faults which shut down the unit. To correct this 'problem' a 'lube cooler' was installed. To cool the lube oil once it reached 100F.
This past summer the lube cooler ran in excess of 150hrs a week.. and the lube tank temp still remained between 100-110F on hot days.

At this point I think you might be right about the bearings. Though should I explain the lube unit/bearing lube reqiurement issue?
I now work on my own... my former employer knew about the lube requirements for these bearings, and knew what the lube unit was rated for. The project manager stated that bearing manufacturers always overstate a bearings required lube flow, and that this should be fine.

Is there any way to prove whether or not he was right? How much should I broach to the plant? I have a good relationship with them, and found a burner wiring issue that my former employer screwed up (300V rated wire for a 600V signal).

I think I may be out of my league in some ways, this is a 700HP motor correct, large costs involved there in repair condiderations.

If all extenuating circumstances, mounting, environment, etc. have been verified OK, and you have the istallation vibration tests THEN I would call in an expert on Vibration Analysis.

You already know there were issues with the lubrication system and may be issues with the bearings, if the "expert" provides any information in that area then you will have what is needed to go to management.

With a 700HP motor I would rather schedule the downtime then have it go down on its own.

The Fan is the #1 Suspect

Russ said:

...Initial inspections indicated that the vibration seemed to be coming from the breech side of the fan. Some insulation on the breech had fallen off...

the fan had been previously sold and returned...

Click to expand...

Why was the fan returned? It will expand with temperature, and that will change the weight distribution if it's not balanced or has a flaw.

If the bearings were bad, I would expect vibration throughout the speed range.

You've tested the system as a whole as much as you can. Now it's time to test individual parts. Get the fan blade balance tested.
Next, the motor. Bearings could have play if worn (is the term runout?). Something could have broken loose inside the motor (armature).
If you continue to run in this condition, you will tear up the motor bearings, if you haven't already. I don't mean to be blunt. A vibration is an indication of something serious. If it's a stress crack in the blade, there could be a catastrophic failure.

Kind of reminds me of when my mom was driving me home when I took my truck into the shop. We are on the highway and get up to 65mph and her car starts to shake. She tells me she needs to get a front-end alignment. I told her, no your tires are out of balance. Then she told me she thinks it’s the alignment because if she drives a little faster it goes away, it only shakes around 65mph. To which I replied, that’s because your tires are out of balance. In the end she needed new tires.

I’m with Keith on this, I would venture that your fan blade is out of balance. So why the change when you installed the air straightener and the higher air temperature? I would guess these items change the amount of pressure exerted on the fan blade itself. I’m no M.E. but I sure would have someone who could check the balance of the blade come in and take a look.

Excuse me, did not mean to ever imply it could not be the fan. As mentioned get someone external to do an analysis and see what they say.

To me it could be either, all other factors known to be ok. Bearings or a fan could offer weird results at different speeds.

Sorry Ron I didn't mean to cut you out.

I'm with Keith and partly Ron on this one.

Tark said:

Kind of reminds me of when my mom was driving me home when I took my truck into the shop. We are on the highway and get up to 65mph and her car starts to shake. She tells me she needs to get a front-end alignment. I told her, no your tires are out of balance. Then she told me she thinks it’s the alignment because if she drives a little faster it goes away, it only shakes around 65mph. To which I replied, that’s because your tires are out of balance. In the end she needed new tires.

I’m with Keith on this, I would venture that your fan blade is out of balance. So why the change when you installed the air straightener and the higher air temperature? I would guess these items change the amount of pressure exerted on the fan blade itself. I’m no M.E. but I sure would have someone who could check the balance of the blade come in and take a look.

Click to expand...

That makes sense about the fan based on temp. The bearings were close to their normal range before I had completed the initial test.
There was strong evidence of turbulence in the duct, and I thought the straightener would resolve that (which I'm fairly certain it has).
However it seems that turbulence wasn't the source of the vibration.

Turbulence is a new factor not mentioned before. The pitch of 1 of the blades could be off. For example, consider the fan blade on the Air Conditioning unit outside a typical home. The blade is soft metal and easily bent. If one of the blades get's bent, the fan will shake. The motor will fail within one season.
Of course we need to scale up with your 700HP system, but the analogy is the same.
It's not clear how you measured turbulence. Doesn't matter - trying to find cause and cure for turbulence is tricky. Checking the fan blade itself will be easier and cheaper.

keithkyll said:

Turbulence is a new factor not mentioned before. The pitch of 1 of the blades could be off. For example, consider the fan blade on the Air Conditioning unit outside a typical home. The blade is soft metal and easily bent. If one of the blades get's bent, the fan will shake. The motor will fail within one season.
Of course we need to scale up with your 700HP system, but the analogy is the same.
It's not clear how you measured turbulence. Doesn't matter - trying to find cause and cure for turbulence is tricky. Checking the fan blade itself will be easier and cheaper.

Click to expand...

I noticed turbulence based upon vibration reading fluctuations during valve changes, and confirmed it by noticing the spiraling dust patterns in the duct.




So, I spoke at length with some motor/fan guys and found out that a good air straightener would reduce the turbulence in the duct. I designed a two part straightener. The first part was to reduce any minor turbulence that occurred from the last turn (after going through the main straightener). It looks like this.


Then, to remove the most turbulence I went with a more aggressive design. A lot of pull through RTOs have a four section air straightener (simple cross design) to remove the turbulence. Due to the extensive vibration and evidence of turbulence I wanted to remove as much as possible, without affecting air flow. I also made part of it removeable.



When I trended the vibration levels this past weekend it was after these were installed.

I am assuming this is a direct coupled fan, not belt driven.

I'm not an expert, but the vibration can be induced by the air stream, or it could be mechanical imbalance, or it could be harmonics.

Higher air temperature does a couple of things. It decreases the fan air flow (due to lower air density) and fan hp. It can cause the bearings to run hotter, which dcreases the oil viscosity, and assuming these are journal bearings the bearing stifness can decrease. This will exacerbate vibration tendencies.